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 Coil or not to coil coax

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Javelin Jim
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PostSubject: Coil or not to coil coax   Coil or not to coil coax Icon_minitimeMon Apr 13, 2020 12:41 pm

Hi,
I’ve another question about setting up a Silver Rod.
I’m mounting it temporarily in my garden on a six foot pole with view to permanently concreting a longer support pole in the future, subject to the neighbours and ‘er indoors permitting.
With this in mind, I was considering making the coax run to the length where I finally want to mount the antenna.
Would coiling the excess length (approx 3-4m)at the antenna end affect the performance?
I could cut it to the length I need now then extend using connectors when I do need the extra length.
What do you guys think?
Cheers Jim
26 CT 3427
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BigTone
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PostSubject: Re: Coil or not to coil coax   Coil or not to coil coax Icon_minitimeMon Apr 13, 2020 6:59 pm

Nope, shouldn't do.
Connectors out there are a point where moisture can get in and ruin your coax
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Cozzmik
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PostSubject: Re: Coil or not to coil coax   Coil or not to coil coax Icon_minitimeTue Apr 14, 2020 8:09 am

While i have no idea if it affects Swr coiling my coax was necessary one time as i wanted to do a similar thing to you..

Swr was fine but i had no idea if it could of been affected coiling it as i had no reference point to check..



Try and see is always my advice....
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Javelin Jim
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PostSubject: Re: Coil or not to coil coax   Coil or not to coil coax Icon_minitimeTue Apr 14, 2020 10:29 am

Hi,
I love waffle...! Waffle is good, I am also a huge waffler...😊
Thanks for the advice guys.
Cheers
Jim
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BigTone
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PostSubject: Re: Coil or not to coil coax   Coil or not to coil coax Icon_minitimeTue Apr 14, 2020 1:41 pm

The choke is to stop RF on the OUTSIDE of the braid (Common mode current)
RF travels on the surface of a conductor and on coax, there are 3 surfaces.
(1) The core (the centre conductor)
(2) The inside of the braid
(3) The outside of the braid

Common mode current (mainly) is a pain but there is a use, the lower leg of a coaxial dipole (T2LT) The choke defines the length.
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Northern Crusader
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PostSubject: Re: Coil or not to coil coax   Coil or not to coil coax Icon_minitimeTue Apr 14, 2020 8:38 pm

NUBSTER wrote:
Do not coil it up either that will effect SWR of the antenna

Sorry but that is wrong and your entire post explaining it all is wrong. If it does alter the SWR it is because you've a poor RF ground so your coax was forming part of the antenna system.

Quote :
anyway we was told when you put up the homebase antenna the coax had to be cut to certain lengths because of the wavelength and to get a perfect SWR

....because the coax was making up for the missing ground plane so altering its length either by cutting it or coiling it up thereby creating a RF choke affected the tuning. Unfortunately next to no CBers understood this so it became an old wives tale you had to have it a certain length for low SWR.

If changing the length or coiling the coax alters the tuning of the antenna then it's telling you you've got a problem with your RF ground at the antenna.


Last edited by Northern Crusader on Tue Apr 14, 2020 8:44 pm; edited 2 times in total
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Northern Crusader
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PostSubject: Re: Coil or not to coil coax   Coil or not to coil coax Icon_minitimeTue Apr 14, 2020 8:40 pm

Javelin Jim wrote:

Would coiling the excess length (approx 3-4m)at the antenna end affect the performance?
I could cut it to the length I need now then extend using connectors when I do need the extra length.
What do you guys think?
Cheers Jim
26 CT 3427

No it shouldn't do but you can put that extra length to use. Find something that's 110mm/4.25" in diameter (ground waste pipe is spot on) and wrap 5 turns of the coax round it as near to the end that goes into the antenna as possible. This will create a RF choke for 11m which will help reduce the risk of common mode RFI which causes interference on received signals, distortion on transmitted ones and the dreaded TVI.

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26TC62
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PostSubject: Re: Coil or not to coil coax   Coil or not to coil coax Icon_minitimeWed Apr 15, 2020 8:15 am

Buy the full length of coax now and coil it up. The reason for not coiling up coax used to be that most people were using horrible RG58 with thin braid which had all sorts of consequences for the behaviour of the current in the coax. One of the effects was that the inner core of the coax started to behave like the turns of an ordinary coil inductor and screw the SWR. Properly screened coax can be coiled, because the inner core is properly screened from the inner core in the next turn of coil and it doesn't "know" it is coiled up.

If you're using a run long enough to go into a garden you probably want to think about RG213 as a minimum anyway, because it is lower loss and much nicer than even the good RG58 some dealers sell now (there is still horrible stuff available -watch out for Ebay etc). You can coil RG213 up happily but try to keep the turns above 8" diameter unless you are making a coil out of it which you never intend to unwind and re-route. You can fix RG213 in turns with a couple of inches radius for permanent fixing.

Avoid in-line connectors and extension pieces if you can - better to overbuy the coax and coil it in the long run than have PL259s and back-to-back SO239s because they are a point of mechanical and electrical failure.
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Javelin Jim
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PostSubject: Re: Coil or not to coil coax   Coil or not to coil coax Icon_minitimeWed Apr 15, 2020 10:58 am

Thanks for all the advice guys.
Really useful information.
I’m expecting the delivery later today of my gear, so the fun will start, so expect more questions...!
I plan to use around 27.500 to take SWR readings. If I get can that low, I’ll write out a frequency/SWR chart to see how the SWR changes above and below 27.500.
Cheers
Jim
26 CT 3427
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Northern Crusader
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PostSubject: Re: Coil or not to coil coax   Coil or not to coil coax Icon_minitimeThu Apr 16, 2020 8:53 pm

Depending on how enthusiastic and how much you want to learn about antennas and experimenting have a look at the nanoVNA analysers on Amazon. They're about £30 and are much more useful than a SWR meter especially when you put up an antenna and its low SWR point is outside the CB bands. With a normal CB SWR meter you'd only be able to test within the range of the CB it's hooked up to so you could see high SWR on every channel and assume the antenna was broken when in fact it wasn't, it was just tuned outside the CB band.

An example is my mobile installs. I have a very good RF ground but unfortunately manufacturers make mobile antennas assuming there won't be a good one which means they need to be made longer. Adjusting the antenna throughout all its adjustment from shortest to longest you'd see the SWR into the red using just a SWR meter. Using the antenna analyser allowed me to see it was actually resonant about 1MHz below the CB band. 3" lopped off with a dremel and it was 1.0:1 right where I wanted it on Ch19.

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43CT016
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PostSubject: Re: Coil or not to coil coax   Coil or not to coil coax Icon_minitimeTue Jun 09, 2020 12:20 pm

Talking of chokes, probably one of the best articles out there is this one:

http://www.karinya.net/g3txq/chokes/

You’ll see from his graphs why the 5 turns of RG213 on 4” pipe is always cited, and if you can wade through all the complex impedance theory, there’s a good explanation of why co-ax chokes can bite you, having a predominantly reactive impedance.

I run his “designs” on all my HF antennas, usually using either 240-31 or 240-43 cores and LMR200 or RG400 depending on what antenna they’re on.

With regards to the OP, I’d echo the answers above and not cut the coax....just coil it loosely out of the way. The old wives tales from the early CB days will never leave us, I feel.

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